In web printing presses, a continuous web of paper is fed through the press. The web travels through various components of the press by passing over both driven and idle rolls. The rolls guide the web through the press, the driven rolls providing motive force and the idler rolls providing position, guidance and direction.
A web printing press typically includes a plurality of processing units. These units may include the printing units, dryer unit, chill unit, and slitter unit. Each printing unit generally prints a separate color onto the web in order to create a color print. For example, in a printing press with four printing units, a first printing unit could print black, a second printing unit could print cyan, a third printing unit could print magenta, and a fourth printing unit could print yellow. The web travels through the nip of each printing unit as the web traverses from print unit to print unit. After the web traverses through the nip of the last printing unit, the web will enter entrance nip rolls of a following processing unit, such as for example, the chill unit.
A pair of nip rolls is a pair of cylindrical rolls arranged with their axes substantially parallel to one another. The circumferential surfaces of the nip rolls are in rolling engagement with each other, the material web passing between the nip rolls in a path approximately perpendicular to the plane of the two parallel axes of the rolls. The traversing surface speed of the web is approximately equal to the circumferential surface speed of the nip rolls.
However, if two pairs of nip rolls rotate such that the circumferential speed of a first pair of nip rolls is different from the circumferential speed of a second pair of nip rolls, or if slippage occurs between the nip rolls and the web, tension or slack may develop in the web. The web is considered to be in tension, or to be taut, when there has been a change in length of 0.1% or more. The web is considered to be slack, or baggy, when the length of a portion of the web extending between two points exceeds a standard amount by, for example, 0.1% or more. Other criteria may of course be used to define a web as being in tension or slack. The criteria described above is based on a 100% safety factor by using the observation that certain types of paper will tear when stretched in excess of 0.2% of its original length.
When a first pair of nip rolls through which the web passes are turning at a faster circumferential speed than a subsequent pair of nip rolls, it is possible that a 0.1% reduction in length of the web will occur, thus resulting in a slack web. When slack exists in the web, the condition may also be referred to as having a baggy web.
Alternatively, when the first pair of nip rolls is rotating more slowly than the subsequent pair of nip rolls, tension will build in the web. Such tension in the web may cause slippage between the rolls and the web. If the tension builds to a high enough level, failure or tearing of the web may occur. If the web fails or tears, the press must be shut down, the torn portion of the web must be removed, and the web must be rethreaded through the press, resulting in expensive down time and loss of operation.
Conventionally, after a web passes through the printing units and through the dryer unit, the web moves through a chill unit and then to a slitter unit. The chill unit is the first unit the web contacts via a nip after exiting the dryer unit of the printing press.
One purpose of the chill unit is to cool down the heated web prior to further processing in the press. The chill unit includes a plurality of chill rolls which operate to cool down the web. The last of the chill rolls is coupled with a further roll to form a pair of chill unit exit nip rolls. Because the dryer unit typically does not contain a set of nip rolls, a second purpose of the chill unit is to pull the web through the dryer unit.
A series of fixed position idler rolls have generally been positioned between the chill unit exit nip rolls and the slitter unit entrance nip rolls located at the entrance to the slitter. These idler rolls rotate freely about fixed axes. Contact with the web provides the motive force to rotate the idler rolls about their axes.
One disadvantage of this design is that the idler rolls relieve tension only by allowing slippage between the rolls and the web. Only a limited amount of tension can be relieved in this way. Another problem is that idler rolls are not capable of taking slack out of baggy webs.
Several attempts have been made to reduce or eliminate baggy webs and broken webs, but none have adequately solved the problem. One attempted solution was to use motor driven tension control systems that attempt to maintain tension control over the web by adjusting motor speed on driven rolls to either increase or decrease the feed rate of specific rolls. Slowing down the motor decreases the feed rate, thereby eliminating a baggy web down stream. Increasing the motor rate reduces down stream tension to prevent a web from breaking. Conceptually, this solution is sound, however, in practice, the response time of these motor driven tension control systems is too great to alleviate the baggy and broken web conditions before problems arise. In addition, known variable speed motor driven tension control systems are inadequate to effectively remove baggy webs or high tension webs during the press start up period when the requirement for dynamic response high.
Dancer rolls have been employed to control tension and baggy webs after the intake feeds of offset printing presses. The intake feed of these systems, located between the roll stand and the first printing unit, controls delivery of the web to the first printing unit to maintain a positive tension in the web as it enters the first printing unit. Dancer rolls have been provided between the roll stand and the intake feed to maintain a constant web tension after the intake feed, or between the intake feed and the first print unit to maintain a constant web tension at the first printing unit. However, dancer rolls typically are not applied to the post chill region of a printing press in part because of concerns about cutoff control. Cutoff control refers to maintaining control over the speed and position of the web, specifically with relation to the location of the printed image so that the web may be cut into signatures in the nonprinted region. A dancer roll causes dynamic changes in the path length of the web. Placing a dancer roll downstream of the print units changes the path length of the web upon which an image has been printed which raises concerns about maintaining cutoff control.